As a farmland manager or farm real estate professional the soils on a farm are one of the most critical elements to consider when valuing a farm for lease or sale.  There are multiple components that make a soils “good or poor” and this is the third in a series of articles that investigates these components – soil structure.

A critical aspect of soil fertility and productivity is soil structure or more specifically, soil aggregation and soil porosity, which are key factors in how air and water exist in the soil.   Soil structure is a description of how the individual soil particles (sand, silt, or clay) bond together and form small clumps known as aggregates.  The soil aggregates are important for multiple reasons with respect to supporting plant growth and soil fertility.

• The formation of soil aggregates, both larger aggregates and microscopic soil aggregates, create pore space which allows oxygen exchange at the surface and provides space for water to be held in the soil. Research shows that the ideal amount of pore space for maximum soil productivity is higher than most would probably guess – approximately 50% pore space by volume.
• Soil porosity allows oxygen to be held in the soil and to be exchanged at the surface with fresh oxygen supporting respiration in the soil. Plant roots can’t live or grow without oxygen for their respiration.  Unlike humans, plants don’t have a circulatory system to move oxygen throughout the plant roots after taking it in above ground.  Roots need direct access to oxygen in the soil to live and grow.  Many beneficial soil microorganisms are aerobic and also need oxygen to live and thrive. This is why low areas that fill with water “drown out the crop” as there is no oxygen getting to the roots.
• Mineralization occurs on the surface of soil aggregates as well as the surface of the bits of organic matter. The more aggregation and surface area there is, the more potential mineralization that can supply nutrients for the crop.
• Water is held in the soil primarily by surface tension. Again, the more aggregation we have in the soil, the more surface area there is to which water can be held.  The force of gravity moves the water down through the soil profile, eventually to the water table.  Surface tension holds some of that water to the aggregates against the force of gravity.  How much water can be held is determined largely by the soil type – sand, silt, or clay.  However, the amount of soil organic matter and microbial life in the soil are also important factors.  Both can help stabilize the aggregates.  For example, some soil microbes produce a sticky protein substance called glomalin which can coat the outside of soil aggregates and significantly contributes to soil stability and water-holding capacity.
• Good aggregation and the resulting soil porosity promotes good drainage when there is excess water in the soil. We know excess water can essentially drown roots in 2-4 days by driving needed oxygen out of the soil, depending on temperature and growth stage.  A robust network of pores allows water to drain lower in the soil toward the water table more quickly.  It can also add to the efficacy of tile drainage by allowing the water to move through the soil to the tile lines with greater ease.
• While soybeans and some other plant species are known for a ‘tap root’ that can push through some amount of soil compaction, grass species like corn and wheat have a more fibrous root system. They have a more difficult time dealing with soil compaction. The network of pore spaces created by soil aggregation provides channels and pathways for robust root growth without the expenditure of a significant amount of the plant’s energy dealing with a more challenging root environment.  It will result in larger root masses which not only drive larger yields but will also build soil microbial life through an increased volume of root exudates that feed the microbes.  In addition, it  ultimately creates more root mass to decay after the crop is harvested that will add to soil organic matter.

It may be useful to point out that soil aggregation is the opposite of soil compaction.  When compaction occurs, it smears the soil particles together, minimizing or eliminating pore space, pushing out the oxygen, and destroying the soil aggregates.  Soil compaction is widely recognized as greatly reducing soil productivity.  Conversely, good soil structure will promote plant growth processes that ultimately drive higher yields.

How do you enhance soil aggregation and porosity?  The most effective soil management practices to increase soil aggregation and porosity are eliminating unnecessary tillage and building organic matter and soil life.  One of the best ways to build organic matter and soil life is adding cover crops to grow between the harvest of the cash crop and the planting of the next year’s cash crop.  Multiple benefits can accrue from cover crops, for example, erosion control, added nutrients, and weed control.  But in addition to these, one of the most important benefits is often keeping living roots in the soil for as much of the year as possible to feed microbial life and add organic matter with more roots to decay.  When combined with reduced tillage over time soil structure improves leading to more porosity, more water holding capacity and more access to nutrients.

Our goal at Halderman is to manage farms to maximize your current income and improve the quality/value of the farm over time.  Increasing the soil’s fertility accomplishes both goals as it enhances current yields and therefore income and builds a more positive reputation for the farm which adds to its value.  Farm management is a long term exercise that requires a vision for the future and patience in the moment for each practice implemented.